1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device using a plurality of thin film transistors as driving elements and a method of fabricating the same.
2. Discussion of the Related Art
In general, organic electroluminescent (EL) devices emit light by injecting electrons from a cathode and holes from an anode into a luminescent layer, combining the electrons and the holes to generate an exciton, and transitioning the exciton from an excited state to a ground state. Contrary to liquid crystal display (LCD) devices, an additional light source is not necessary for the organic EL devices because the transition of the exciton between two states causes light to be emitted from the luminescent layer. Accordingly, size and weight of the organic EL devices can be reduced. Since the organic EL devices have low power consumption, superior brightness, and fast response time, the organic EL devices are incorporated in various consumer electronic products, such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabrication of the organic EL devices is simple, the production cost for the organic EL devices is lower than the LCD devices.
Organic EL devices may be categorized into two groups: passive matrix organic EL devices and active matrix organic EL devices. Although the passive matrix organic EL devices have a simpler structure and are disposed using simple fabricating processes, the passive matrix organic EL devices have some disadvantages. The passive matrix organic EL devices require relatively high amounts of power to operate the devices, and display sizes of the passive matrix organic EL devices are limited by their structures. In addition, as a total number of conductive line increases, aperture ratios of the passive matrix organic EL devices decrease. In contrast, the active matrix organic EL devices have a high luminescent efficiency and can produce high-quality images for increased-size displays using relatively low power.
FIG. 1 is a schematic cross-sectional view of an organic electroluminescent device according to the related art. In FIG. 1, an array unit 14 including a thin film transistor (TFT) “T” is formed on a first substrate 12. A first electrode 16, an organic luminescent layer 18, and a second electrode 20 are sequentially disposed on the array unit 14, wherein portions of the organic luminescent layer 18 may separately display red, green, and blue colors for each pixel region “P.” In general, separate organic materials are provided to emit light corresponding to the colors red, green, and blue in each pixel region “P” provided in the organic luminescent layer 18. An organic EL device is encapsulated by attaching the first substrate 12 and a second substrate 28, which includes a moisture absorbent material 22, with a sealant 26. The moisture absorbent material 22 eliminates any moisture and oxygen that may penetrate into a capsule of the organic luminescent layer 18. A portion of the second substrate 28 is etched to create a room for the moisture absorbent material 22. The etched portion is filled with the moisture absorbent material 22 and is fixed by a holding element 25.
FIG. 2 is an equivalent circuit diagram showing a single pixel region of an organic electroluminescent device according to the related art. In FIG. 2, a switching element “TS” is connected to a gate line 34 and a data line 36 intersecting each other and a driving element “TD” is connected to the switching element “TS.” The driving element “TD” is a positive (P) type thin film transistor (TFT) and a storage capacitor “CST” is connected to a driving gate electrode 40 and a driving source electrode 42. In addition, an organic electroluminescent (EL) diode “E” is connected to a driving drain electrode 44 and a power line 46 is connected to the driving source electrode 42 of the driving element “TD.”
When a gate signal is applied to a switching gate electrode 38 of the switching element “TS” through the gate line 34, the switching element “TS” is turned on and a data signal of the data line 36 is stored in the storage capacitor “CST” through the switching element “TS.” The data signal is also applied to the driving gate electrode 40, thereby turning the driving element “TD” on. Thus, a current of the power line 46 flows through a channel of the driving element “TD” and is transmitted to the organic EL diode “E.” As a result, the organic EL diode “E” emits light in proportion to the current density. The organic EL diode “E” is a current driving type which is implemented to received fixed power voltage supplied from the power line 46, and the brightness of light is controlled by the current. Since the driving element “TD” is driven by charges stored in the storage capacitor “CST,” the current through the organic EL diode “E” is persistent until a next data signal is applied even when the switching element “TS” is turned off. As a result, light is emitted from the organic EL diode “E” until a data signal of the next frame is applied.
The switching element “TS” and the driving element “TD” are formed of an amorphous silicon thin film transistor (TFT) or a polycrystalline silicon TFT. The amorphous silicon TFT may be fabricated easier than the polycrystalline silicon TFT. When the amorphous silicon TFT is used as the switching element “TS” and the driving element “TD,” an increased width to length (W/L) ratio of the amorphous silicon TFT is required for an increased current density. However, the amorphous silicon TFT having the increased W/L ratio may be deteriorated due to a stress resulting from the increased current density. Specifically, since a direct current (DC) bias is continuously applied to the driving element “TD,” characteristics of the driving element “TD” may vary widely. Accordingly, the deterioration of the driving element “TD” causes a dot defect of an organic EL device.
Moreover, in an organic EL device according to the related art, an array unit and an organic EL diode are formed on a first substrate, and an additional second substrate is attached to the first substrate to encapsulate the organic EL device. However, when the array unit and the organic EL diode are formed on single substrate in this manner, production yield of the organic EL device is determined by a multiplication of the TFT's yield and the organic EL diode's yield. Since the organic EL diode has a relatively low yield, overall production yield of the EL device is limited by the organic EL diode's yield. For example, even when a TFT is well fabricated, an organic EL device using an organic luminescent layer of about 1000 Å thickness may be viewed as a poor example when an organic EL layer is defected. This results in loss of materials and increase in production costs.
Furthermore, organic EL devices are classified into bottom emission types and top emission types according to an emission direction of light used for displaying images via the organic EL devices. Bottom emission type organic EL devices have the advantages of high encapsulation stability and high process flexibility. However, the bottom emission type organic EL devices are ineffective for high resolution devices because they have poor aperture ratios. In contrast, top emission organic EL devices have an increased expected life span because they can be designed more easily and have an increased aperture ratio. However, top emission type organic EL devices generally include an organic EL layer having the cathode formed thereon. As a result, transmittance and optical efficiency of the top emission type organic EL devices are reduced because of a limited number of materials that may be selected. If a thin film-type passivation layer is disposed to prevent a reduction of the light transmittance, the thin film-type passivation layer may fail to prevent infiltration of exterior air into the device.